The present invention relates to a workpiece gripping apparatus, especially for an indexing chuck.
Indexing chucks have been known for about 50 years, and allow various faces of a workpiece to be machined on a lathe without having to release the workpiece from the chuck and to reclamp it. With typical known indexing chucks, the top jaw, which is attached to the clamping piston of the indexing chuck, moves only in the vertical direction to provide clamping for the workpiece. However, since the bottom jaw is fixed in the vertical position, the two jaws are not able to self-center a workpiece in the vertical direction. Such indexing chucks employ one master clamping jaw, making them single jaw clamping chucks. In order to properly center the workpiece in such an indexing chuck, one relies on the symmetrical and concentric nature of the workpiece itself. Unfortunately, although different casting lots of workpieces generally have similar centerlines or a similar deviation from the centerline, these centerlines are often not concentric. This is compensated for by placing under the indexing jaw a washer that has been ground to the size required for raising the indexing jaw a sufficient amount for bringing the workpiece on centerline. Although placing a ground washer between the indexing jaw and the indexing master jaw of the chuck works well when handling a batch of workpieces having similar characteristics of size and concentricity, when the workpiece characteristics are not consistent from one piece to another, a single ground washer will not be sufficient to bring workpieces having great size variations onto a common centerline for machining. This is a common limitation of the known indexing chuck technology, and is one of the great shortcomings of the use of a single jaw clamping chuck.
It is therefore an object of the present application to make it possible to self-center workpieces, and in particular workpieces having unacceptably large external differences that prevent such workpieces from being properly clamped on a common centerline in a single jaw indexing chuck without the need to constantly change a centering washer mounted under the indexing jaw.